This invention relates to a semiconductor device, and more particularly to a power transistor which is contained in a high-output semiconductor integrated circuit (hereinbelow called "power IC").
In a power transistor in the power IC of an audio output circuit or the like, a plurality of emitter regions are arranged within a single base region in order to derive a high output current. In such a power transistor, ballast resistances are inserted between the respective emitter regions and the corresponding emitter input terminals so that emitter currents may flow uniformly through the respective emitter regions. Also, in order to make the occupied area of the whole power transistor small, it has been proposed to form each ballast resistance as a diffused resistance region with one end continuous with the original emitter region and the other end connected to an emitter input electrode.
Although such a power transistor as discussed above has a relatively small occupied area, it has the disadvantage that current crowds locally in the part of the ballast resistances proximate to the input electrodes, resulting in the breakdown of the transistor. Upon studying this breakdown, the inventor has found it is ascribable to the fact that the ballast resistance region proximate to the input electrode also effects a transistor operation. That is, in these studies it has been revealed that current becomes liable to crowding in a parasitic transistor whose emitter is the ballast resistance region, and that the local current crowding occurs in this part of the region with the result that the power transistor breaks down.